Reducing equivalents such as nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH) are important coenzymes for enzymatic redox reactions such as oxidoreducatase reactions and are found in all living cells. It is generally accepted that the NADPH pool is considerably smaller than the pool of NADH (G. N. Bennett & San, 2009). In E. coli grown on glucose sugar the pool of NADH is over 20 times larger than the NADPH pool (B. D. Bennett et al., 2009). This low NADPH availability limits many biosynthetic reactions and bioconversions especially in fermentation processes (R Poulsen et al., 2005). The preference of enzymes for NADPH can limit the production of a desired product (G. N. Bennett & San, 2009). This is a problem when engineering new reactions and pathways into a microorganism and is one of the major hurdles for the generation of efficient production platforms of compounds including biofuels, chemicals, amino acids or vitamins (Chemler, Fowler, McHugh, & Koffas, 2010).
Nevertheless, metabolic engineering has been successfully demonstrated for production of a wide range of fuels and chemicals (Peralta-Yahya & Keasling, 2010) by limiting, avoiding or bypassing NADPH dependent reactions where possible. Alternatively, energy-consuming transhydrogenases have been used that interconvert between NADH and NADPH pools. Another strategy to achieve successful metabolic engineering is elimination of competing NADPH dependent reactions. Despite these advances, such novel strategies are often pursued at the expense of production yields and/or growth rates (Auriol, Bestel-Corre, Claude, Soucaille, & Meynial-Salles, 2011). Further, they only become possible by extensive engineering work with multiple modifications (S. M. Ma et al., 2011). Thus these efforts have been limited only to genetically tractable organisms such as Escherichia coli and Saccharomyces cerevisiae (Peralta-Yahya & Keasling, 2010). These organisms are limited as they feed only on sugar. Accordingly, their commercial use and viability suffers from the significant drawbacks around land-use, food-security, volatility of supply and environmental issues.
Carboxydotrophic Clostridia offer an alternative to E. coli and S. cerevisiae and are able to grow on waste gases and syngas. There are a few examples of recombinant carboxydotrophic clostridia which have a limited number of modifications (Schiel-Bengelsdorf & Dürre, 2012). All known examples use NADH-dependent reactions.
It is an object of the invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide the public with a useful choice.